1. Field of the Invention
The present invention relates to a current mirror circuit for use in, e.g. a motor control circuit for controlling a spindle motor employed in a hard disk drive, and a control system to which the current mirror circuit is applied.
2. Description of the Related Art
A spindle motor for rotating a disk at high speed is provided in, e.g. a conventional hard disk drive (HDD). The movement (rotation) of the spindle motor is controlled by a current control type motor control apparatus. FIG. 1 shows an example of the current control type motor control apparatus. In general, a current control type motor control apparatus 1 comprises a current control circuit 2, a phase switch signal generating circuit 3 and a driver circuit 4. A motor driving signal is supplied to phase coils 5a to 5c of a three-phase motor 5 which is to be controlled.
The current control circuit 2 is connected to a power supply (power supply voltage VCC) and supplies a drive current to the driver circuit 4 in accordance with a current control signal (target current value) supplied from a control unit such as a CPU (Central Processing Unit). As is shown in FIG. 1, the current control circuit 2 includes a current detection resistor 2a, a current control transistor 2b and an operational amplifier 2c. The operational amplifier 2c compares a signal (detection voltage) representing a drive current value detected by the resistor 2a with a current control signal, and applies a base voltage corresponding to an error between the drive current value and the target current value to the base of the current control transistor 2b. Thereby, the operation of the current control transistor 2b is controlled, and a desired driver current is supplied to the driver circuit 4.
The driver circuit 4 switches the direction of currents flowing to, e.g. the phase coils 5a to 5c of the three-phase motor 5 in accordance with phase switch signals supplied from the phase switch signal generating circuit 3. FIG. 2A shows an example of the driver circuit 4. As is shown in FIG. 2A, the driver circuit 4 is an H-bridge circuit having MOS (Metal Oxide Semiconductor) type field effect transistors (MOS-FETs) 4a to 4f. The driver circuit 4 supplies drive currents to the phase coils 5a to 5c, with the current control circuit 2 employed as a current source. The MOS-FETs 4a to 4f function as switches. Specifically, MOS-FETs 4a, 4c and 4e function as switches between the current source 2 and the phase coils 5 of the motor, and MOS-FETs 4b, 4d and 4f function as switches between the phase coils 5 and the ground.
For example, when a drive current is supplied from the phase coil 5a to the phase coil 5b, the phase switch signal generating circuit 3 outputs phase switch signals to the driver circuit 4 to turn on the MOS-FETs 4a and 4d. FIG. 2B illustrates schematically the case where the drive current is supplied from the phase coil 5a to the phase coil 5b. A composite resistor constituted by series-connecting the phase coils 5a and 5b is shown as a load resistor 5ab. Thus, a series circuit, in which a drive current flows from the current control circuit 2 to the FET 4d via the FET 4a and load resistor 5ab (phase coils 5a and 5b), is constituted.
With the above structure, the motor control apparatus 1 supplies a desired drive current via a desired path to the phase coils 5 of the motor to be controlled.
On the other hand, in these years, various electronic devices having the aforementioned motor control apparatus have decreased in size and operational voltage more and more. In particular, the aforementioned HDD is applied to a storage apparatus such as a small-sized personal computer or a portable computer. It is expected that 2.5-inch media (recording media) widely used at present will be replaced with 1.8-inch media and the operation voltage will decrease from 5 V to 3 V. with this tendency of smaller size and lower operational voltage, the following problems will occur in the aforementioned current control type motor control apparatus 1.
Since the current detection resistor 2a of current control circuit 2 is connected in series with the driver circuit 4 (motor 5 including phase coils 5a to 5c), a voltage loss occurs due to current detection. Moreover, since the current control transistor 2b is provided, a voltage loss occurs in a VCE saturation voltage (collector-emitter voltage) at the transistor 2b. A description will now be given of the case where a drive current flows from the phase coil 5a to the phase coil 5b, as shown in FIG. 2B.
With reference to FIG. 2B, suppose that a voltage drop due to the current detection resistor 2a is V1, and a saturation voltage across the collector and emitter of the current control transistor 2b is VCE. In addition, suppose that a drain-source resistance at the time of turn-on of the MOS-FETs 4a and 4d of driver circuit 4 is RON and a composite resistor 5ab is Rcoil. In this case, a drive current (I) supplied to the motor (phase coils 5a and 5b) is given by ##EQU1##
Thus, when the spindle motor requiring a large drive current is actuated, a voltage loss (V1 due to current detection resistor 2a; collector-emitter saturation voltage VCE of current control transistor 2b) will occur. Consequently, a driving torque of the spindle motor decreases.
Specifically, suppose that the power supply voltage is 5 V, the current detection resistance 2a is 0.5.OMEGA., the VCE saturation voltage is 0.3 V, the resistance of MOS-FET 4a, 4d is 1.0.OMEGA., and the composite resistance (coils 5a and 5b) of the motor 5 is 5.0.OMEGA.. Under these conditions, a maximum value I of a drive current capable of being supplied to the motor 5 is 723 mA. However, for example, because of the aforementioned tendency of lower operational voltage, if the power supply voltage is 3.3V, the maximum value I of the drive current is 461 mA, and the maximum current value decreases by about 36%, as compared to the case where the power supply voltage is 5 V. Accordingly, the driving torque decreases. Thus, when the motor such as a spindle motor is driven at low voltage, a current control system with high driving efficiency is needed.
In addition, a voltage drop due to the current detection resistor 2a is proportional to the magnitude of drive current. In order to reduce the aforementioned voltage loss, the drive current may be increased by decreasing the resistance value of the current detection resistor 2a. However, in this case, the precision of current value detection by the current detection resistor 2a deteriorates. As a result, an exact current value of a current to be detected cannot be detected. That is, a signal. (representing a detection voltage) having a predetermined ratio to the magnitude of drive current cannot be output.